1. Field of the Invention
This invention relates to power supplies, and more particularly to power supplies which include a current-mode control system for sensing the inductor primary winding current level.
2. Description of the Prior Art
Prior art current-mode controlled power supplies utilize various types of feedback systems to limit the peak current flow through the primary winding of a coupled inductor. Generally a resistor or a current transformer is coupled in series with a switching transistor to sense the current level in the primary winding. Both of these current level sensing elements induce losses and reduce the power supply efficiency level.
Prior art current-mode controlled power supplies use various techniques to transition the switching transistor from a conductive state into a non-conductive state when the primary winding current sensing circuit detects that the maximum desired primary winding current level has been reached. In accomplishing this switching function, some power supply circuits remove the positive bias voltage from the switching transistor, while other circuits incorporate a negative bias supply which initially provides a low level bias voltage which increases magnitude during the output capacitor charging cycle. When the negative bias voltage level is low, the switching circuit is comparatively unstable and inefficient. Toward the end of the capacitor charging cycle, the negative bias voltage increases to a level which exceeds the switching transistor avalanche breakdown voltage producing highly inefficient operation and excessive heat dissipation.
For the reasons discussed above, prior art current-mode controlled power supplies typically operate at efficiency levels of from between fifty to about seventy percent.
U.S. Pat. No. 4,321,507 (Bosnak) discloses a strobe power supply utilizing a coupled inductor having a primary winding, a secondary winding, a feedback winding and a drive winding. Current transformer T.sub.2 is inserted in the switching transistor emitter circuit to sense the primary winding current level so that the switching transistor can be switched from the conductive state into the non-conductive state at a predetermined maximum current level. This current-mode controlled power supply controls the operation of the switching transistor by applying and removing a positive bias voltage. It does not provide negative bias voltage to the switching transistor in the non-conductive state.
U.S. Pat. No. 3,417,306 (Knak) discloses a strobe power supply circuit including an inductor having primary, secondary and feedback windings. The feedback winding is coupled to provide a negative bias voltage to the switching transistor when the switching transistor is in the non-conductive state. The magnitude of the negative bias voltage varies with changes in the power supply output voltage.
In an article entitled "Current-Sensing IC Improves Regulation of Power Supplies", Electronic Products, June 17, 1985, pages 77-82, Glenn Fritz describes a current-mode controlled power supply utilizing a specialized integrated circuit in the power supply control logic. In the description accompanying FIG. 7 at page 82 of this article, the author indicates that the FIG. 7 power supply operates at a seventy percent efficiency level at full load. FIG. 1 at page 78 of this article depicts a power supply utilizing a current sensing resistive feedback element. At page 80, right hand column, the author indicates that a current transformer may be utilized as a primary winding current level measuring device.